Optoelectronic devices include photovoltaic (PV) devices (solar cells), photodetectors, and like devices, as well as electroluminescent (EL) devices such as light-emitting diodes (LEDs) and laser diodes (LDs). A PV device generates electric power when electromagnetic radiation is incident upon its active layer. The power may be utilized by a resistive load (e.g., battery, electrical power-consuming device, etc.) connected across the PV device. When sunlight is utilized as the source of incident electromagnetic radiation, the PV device may be referred to as a solar cell. A photodetector operates similarly to a PV device, but is configured to detect the occurrence of incident light and/or measure the intensity, attenuation or transmission of incident light and thus may be utilized in various optical sensing and imaging applications. The operation of a photodetector typically entails the application of an external bias voltage whereas the operation of a PV device does not. Moreover, a photodetector often detects only a narrow range of wavelengths (e.g, an infrared (IR) detector or ultraviolet (UV) detector), whereas a PV device is typically desired to be responsive to as wide a range of wavelengths as possible for maximum generation of electrical power. It would, however, be desirable to provide a photodetector that is capable of detecting a broad range of wavelengths, such as from visible to IR or UV to IR.
A photodetector, PV device or related optoelectronic device may be based on a junction formed by a pair of two different types of semiconductors (e.g., an n-type and a p-type material, or an electron acceptor and an electron donor material). When a photon's energy is higher than the band gap value of the semiconductor, the photon can be absorbed in the semiconductor and the photon's energy excites a negative charge (electron) and a positive charge (hole). For the excited electron-hole pair to be successfully utilized in an external electrical circuit, the electron and the hole must first be separated before being collected at and extracted by respective opposing electrodes. This process is called charge separation and is required for photoconductive and photovoltaic effects to occur. If the charges do not separate they can recombine and thus not contribute to the electrical response generated by the device.
In photodetectors, PV devices and related optoelectronic devices, a key figure of merit is quantum efficiency, which includes both external quantum efficiency (EQE) and internal quantum efficiency (IQE). EQE corresponds to the percentage of total incident photons that are converted to electrical current, and IQE corresponds to the percentage of total absorbed photons that are converted to electrical current. Another performance-related criterion is the signal-to-noise (S/N) ratio of the device, which generally may be maximized by maximizing the EQE and minimizing the dark current. In addition, charge carrier mobility within the constituent layers is a key material property that affects the performance of the device. Charge carrier mobility describes the velocity of a charge carrier in the presence of an electric field. A larger value of mobility means that charge carriers move more freely and can be extracted from the device more efficiently. This results in higher device performance as compared devices with lower charge carrier mobility. A related property is exciton diffusion length, which describes the average distance that an exciton (a bound electron-hole pair) will travel before the charge carriers recombine. In a photodetector or related device where excitons play a significant role, a larger value means that there is a higher probability that photogenerated excitons will reach a charge separation region prior to recombination, and also leads to a higher device performance as compared to a photodetector device with a lower exciton diffusion length. While mobility and exciton diffusion are separate properties, their values are affected by similar material attributes. For example, defects, charge trapping sites, and grain boundaries all inhibit carrier transport and result in lower mobility as well as lower exciton diffusion length. While enhanced mobility is discussed throughout this document, it is understood that similar results are obtained for enhanced exciton diffusion length.
Conventionally, photodetector devices and other optoelectronic devices have utilized bulk and thin-film inorganic semiconductor materials to provide p-n junctions for separating electrons and holes in response to absorption of photons. In particular, electronic junctions are typically formed by various combinations of intrinsic, p-type doped and n-type doped silicon. The fabrication techniques for such inorganic semiconductors are well-known as they are derived from many years of experience and expertise in microelectronics. Detectors composed of silicon-based p-n junctions are relatively inexpensive when the devices are small, but costs scale approximately with detector area. Moreover, the bandgap of Si limits the range of IR sensitivity to ˜1.1 μm. Because silicon has an indirect bandgap and is a relatively inefficient absorber of photons, there is a wide distribution of absorption lengths as a function of wavelength, making it difficult to produce detectors that are simultaneously efficient in the UV and the IR. Group III-V materials such as indium-gallium-arsenide [InxGayAs (x+y=1, 0≦x≦1, 0≦y≦1)], germanium (Ge) and silicon-germanium (SiGe), have been utilized to extend detection further into the IR but suffer from more expensive and complicated fabrication issues. Other inorganic materials such as AlxGayInzN (x+y+z=1, 0≦x≦1, 0≦y≦1, 0≦z≦1), SiC, and TiO2 have been used for more efficient UV detection but also suffer from complex fabrication and cost issues.
More recently, optoelectronic devices formed from organic materials (polymers and small molecules) are being investigated, but have enjoyed limited success as photodetectors. The active region in these devices is based on a heterojunction formed by an organic electron donor layer and an organic electron acceptor layer. A photon absorbed in the active region excites an exciton, an electron-hole pair in a bound state that can be transported as a quasi-particle. The photogenerated exciton becomes separated (dissociated or “ionized”) when it diffuses to the heterojunction interface. Similar to the case of inorganic PV and photodetector devices, it is desirable to separate as many of the photogenerated excitons as possible and collect them at the respective electrodes before they recombine. It can therefore be advantageous to include layers in the device structure that help confine excitons to charge separation regions. These layers may also serve to help transport one type of charge carrier to one electrode, while blocking other charge carriers, thereby improving the efficiency of charge carrier extraction. While many types of organic semiconductor layers can be fabricated at relatively low-cost, most organic semiconductor layers are not sufficiently sensitive to IR photons, which is disadvantageous in IR imaging applications. Moreover, organic materials are often prone to degradation by UV radiation or heat.
Even more recently, quantum dots (QDs), or nanocrystals, have been investigated for use in optoelectronic devices because various species exhibit IR sensitivity and their optoelectronic properties (e.g., band gaps) are tunable by controlling their size. Thus far, QDs have been employed in prototype optoelectronic devices mostly as individual layers to perform a specific function such as visible or IR emission, visible or IR absorption, or red-shifting. Moreover, optoelectronic devices incorporating QDs have typically exhibited low carrier mobility and short diffusion length.
A photodetector may form the basis of an imaging device such as, for example, a digital camera capable of producing still photographs and/or video streams from an observed scene. The imaging device in such applications typically includes a light-sensitive focal plane array (FPA) composed of many photodetectors and coupled to imaging electronics (e.g., read-out chips). The photodetector of a typical digital camera is based on silicon technology. Silicon digital cameras have offered outstanding performance at low cost by leveraging Moore's Law of silicon technology improvement. The use of silicon alone as the light-absorbing material in such cameras, however, limits the efficient operation of these cameras in the infrared spectrum. Silicon is therefore not useful in the portion of the electromagnetic spectrum known as the short-wavelength infrared (SWIR), which spans wavelengths from ˜1.5 to 2.5 μm. The SWIR band is of interest for night vision applications where imaging using night glow and reflected light offers advantages over the longer thermal infrared wavelengths. Similarly, the typical IR-sensitive imaging device composed of, for example, InGaAs, InSb, or HgCdTe is not capable of also performing imaging tasks in the visible and UV ranges. Hence the requirement in many imaging systems for both daytime and nighttime imaging has resulted in the use of multi-component systems containing silicon-based imagers and separate specialized IR imagers. The necessity of utilizing multiple technologies increases costs and complexity. Moreover, SWIR imaging is useful, for example, in military surveillance and commercial security surveillance applications and is considered to have technological advantages over MWIR and LWIR imaging, but thus far has been limited to use in high-performance military applications due to the high costs associated with traditional design and fabrication approaches. Additionally, while FPAs exhibiting good sensitivity to incident IR radiation have been developed based on a variety of crystalline semiconductors, such FPAs conventionally have been required to be fabricated separately from the read-out chips. Conventionally, after separately fabricating an FPA and a read-out chip, these two components are subsequently bonded together by means of alignment tools and indium solder bumps, or other flip-chip or hybridization techniques. This also adds to fabrication complexity and expense.
There is an ongoing need for photodetector devices with improved material properties and performance-related parameters such as more efficient charge separation, greater charge carrier mobility, longer diffusion lengths, higher quantum efficiencies, and sensitivity tunable to a desired range of electromagnetic spectra. There is also a need for lower cost, more reliable and more facile methods for fabricating such photodetector devices, as well as improved integration of the sensing elements with the signal processing electronics, improved scalability for large-area arrays, and applicability to curved, flexible or foldable substrates. There is also a need for photodetector devices that exhibit a sensitivity spanning a broad spectral range, such as both visible and IR or UV, visible and IR, to enable simultaneous detection in these ranges by a single photodetector device.